The adrenergic nervous system plays a major role in the innervation of heart, blood vessel and smooth muscle tissue. Compounds capable of interacting with receptor sites within the adrenergic nervous system can initiate a variety of physiological responses. including vasoconstriction, vasodilation, and increased or decreased heart rate (chronotropic), contractility (inotropic) and metabolic activity. In the past, various adrenergic compounds have been employed to affect these and other physiological responses. However, many adrenergic compounds do not possess significant selectivity to enable desirable interactions with adrenergic receptor sites. That is, these adrenergic compounds do not demonstrate a high degree of specificity for differing receptor types within the adrenergic nervous system in order to obtain a desired physiological response separate from other possible, and perhaps less desirable, responses of the system.